Dominic Thiem reached the semifinals at Indian Wells Masters without hitting a ball as injured Gael Monfils withdrew.
Monfils pulled out shortly before his scheduled quarter-final against Thiem with left Achilles tendon trouble.
Oft-injured Monfils said he'd felt discomfort in his ankle for a couple of days and had been receiving treatment, but the pain was worse after his fourth-round win over Philipp Kohlschreiber on Wednesday night.
"I tried a little jog this morning, and I couldn't really actually push on it," he said. "Then we taped it. Then I hit a little bit tonight, and I felt that I cannot be 100 percent. I have this pain, and it wasn't the right choice to play."
He said an ultrasound exam confirmed severe inflammation, and he wasn't yet sure if it would prevent him from playing the upcoming Miami Masters.
"Very disappointing, to be honest," said Monfils, who was enjoying a 15-3 start to the season. "Because I was playing great tennis, feeling strong."
Thiem is through to the semi-finals at Indian Wells for the first time, having reached the quarters once before in 2017.
"I was really surprised," Thiem said, who was waiting to be taken to the court when he realized there was a problem.
"I'm really sorry for him. Obviously we are both playing well here. He's having a great season. I'm picking up my form again. So I think it would have been very tough but also a very fun match for us, for the crowd. So it's a pity," said Thiem.
